Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a sonde is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long wireline cable that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated, thereby enabling measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations.
Among the available wireline and LWD tools are a variety of electrode-based tools to evaluate electromagnetic properties of a formation. For example, electrode-based tool measurements are often used in the oil and gas industry to evaluate formation resistivity. Example tools of this type are laterolog, laterolog array, and micro-spherically focused log tools. Obtaining an electrode-based tool measurement may, for example, involve placing electrodes in contact with the formation. Some electrodes inject current into the formation, while other electrodes measure voltages generated by the flow of current. The equations that relate voltages and currents for electrode-based tool measurements can be written as a linear system of equations of the form V=IR. Often assumptions are made to simplify the processing/interpretation of electrode-based measurements. An example assumption may be that there is an infinite input impedance internal to the tool between active or inactive excitation electrodes and a return electrode. Such assumptions may decrease the accuracy of electromagnetic formation properties derived from electrode-based tool measurements.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.